Daniel Ryan 15, was last seen on Cardigan Road, Bridlington at 1pm on Monday.
It is now thought that he is in the company of Larissa Rowe, also 15 and from Bridlington, who was reported as missing to police on Monday, May 10.
Humberside Police are concerned for Daniel's safety as it is believed he requires medical attention.
Officers have so far been unable to locate or speak with Daniel, despite extensive searches.
Daniel is described as 5’11” tall, of a slim build, with shoulder long dark brown hair and brown eyes. He is believed to be wearing a black Slipknot hoodie, black leather jacket, black ripped jeans and black lace up boots.
Larissa is described as 5’5” tall, with short straight brown hair and brown eyes. She is believed to be wearing a beige hoodie, blue ripped jeans, and black nike trainers with a white pattern.
A spokesperson for Humberside Police said: "Daniel and Larissa, if you see this, you are not in any trouble, we just want to make sure you’re okay and would ask that you get in touch."
Anyone who sees Danielle and Larissa or knows where they are should contact police on 101 quoting log 425 of May 10.
Humberside Police are currently unable to release a photo of Larissa Rowe.